Natural resources such as gas, oil, and water residing in a subterranean formation or zone are usually recovered by drilling a wellbore down to the subterranean formation while circulating a drilling fluid through the drill pipe and the drill bit and upwardly through the wellbore to the surface. The drilling fluid serves to lubricate the drill bit and carry drill cuttings back to the surface. After the wellbore is drilled to the desired depth, the drill pipe and drill bit are typically withdrawn from the wellbore while the drilling fluid is left in the wellbore to provide hydrostatic pressure on the formation penetrated by the wellbore and thereby prevent formation fluids from flowing into the wellbore.
The next operation in constructing the wellbore usually involves running a string of pipe, e.g., casing, in the wellbore. During this operation, the drilling fluid may remain relatively static in the wellbore for up to 2 weeks, depending on the depth of the wellbore and the difficulty in running the string of pipe in the wellbore. While drilling fluids do not set into hard masses, they do increase in gel strength over time when they are static. As such, the gel strength of the drilling fluid may build up during this period of time. Next, primary cementing is typically performed whereby a cement slurry is pumped down through the string of pipe and into the annulus between the string of pipe and the walls of the wellbore to allow the cement slurry to set into a hard mass (i.e., sheath), and thereby seal the annulus. The cement slurry ideally displaces the drilling fluid from the annulus. However, if the drilling fluid has developed gel strength, portions of the drilling fluid are bypassed by the cement slurry such that areas of the annulus contain drilling fluid rather than the preferable cement slurry. Since the drilling fluid is not settable, complete zonal isolation of the subterranean formation is not achieved, and formation fluids can undesirably flow into and through the wellbore.
To overcome the problems associated with incomplete drilling fluid displacement, so-called settable spotting fluids have been developed to displace drilling fluids from wellbores prior to primary cementing. They have rheological properties to provide for such displacement even if the drilling fluids have relatively high gel strengths. Further, they typically can set over time into a rigid mass having compressive strength, thereby isolating subterranean formations even if the settable spotting fluids are bypassed during primary cementing.
Traditional settable spotting fluids (SSFs) are water-based fluids comprising blast furnace slag, which is the blast furnace by-product formed in the production of cast iron. Such water-based SSFs are incompatible with oil-based drilling fluids and can occasionally cause hole-stability problems or damage to the permeability of producing subterranean formations. Further, the slag-containing SSFs are limited to wellbores having bottomhole static temperatures (BHSTs) of 90° F. or less, i.e., the temperatures at which they slowly set. Further, the slag-containing SSFs prematurely set if they become mixed with cement. To prevent such premature setting, a strong set retarder is typically added to the SSF, and a spacer fluid is commonly inserted between the SSF and the cement slurry.
Oil-based SSFs have been developed that can be used in wellbores drilled with oil-based fluids and that avoid the problems of the water-based SSF's described above. However, such oil-based SSF's contain hydrocarbons that are detrimental to the environment. A need therefore exists for an environmentally friendly SSF that can be used in wellbores having BHSTs greater than 90° F. and that do not prematurely set when exposed to cement.